The present invention relates in general to imaging objects and/or areas. More particularly, the invention provides a method and system for imaging based on mutual coherent radiometry. Merely by way of example, the invention is described as it applies to satellite and airborne surveillance, but it should be recognized that the invention has a broader range of applicability.
Correlation radiometry is related to interferometric radiometry. Interferometric radiometry is a terminology that is familiar to the community of radio astronomy and optical sciences. One of the earliest known experiments of interferometry is the Young's double slit experiment, which demonstrated the interference pattern produced by monochromatic light. Such double slit experiment as well as others that followed largely established the construct of using an interference fringe pattern as a measure for characterizing the mutual coherence between two signals, including thermal emission signals. When the thermal emission signals to be characterized are polychromatic, interference fringe patterns do not form clearly, therefore other methods or apparatus may be more suitable for characterize the mutual coherence of polychromatic signals.
Thermal emission signals are emissions originated from sources of incoherent electromagnetic radiation. The statistical properties of thermal emission may be utilized to form an image of a scene of interest. A statistical measure that underlies the principle of interferometric image formation is the mutual coherence function. Conventional aperture synthesis imaging systems usually rely on the Van Cittert-Zernike theorem, which is based on the quasi-monochromatic assumption. The quasi-monochromatic assumption usually requires the conventional aperture synthesis imaging systems to use narrow band signals. The Van Cittert-Zernike theorem is a quasi-monochromatic formulation of the mutual coherence function, and the formulation leads to methods and systems for image formation that usually require the imaging systems to collect Γ(U,V) data to fill a (U,V) plane. The data in the (U,V) plane are usually defined assuming narrow band signals. For example, the principle of aperture synthesis imaging has been applied to observing celestial stars and other objects. Conventional aperture synthesis imaging systems general operate in accordance with the Van Cittert-Zernike theorem and the quasi-monochromatic assumption.
The method of conventional aperture synthesis generally requires that a significant portion of the (U,V) plane be covered with Γ(U,V) data, or interferometric data. For example, to obtain acceptable image quality, a conventional aperture synthesis imaging system generally has to collect a sufficient amount of lower spatial frequency Γ(U,V) data, which corresponds to the central area of the (U,V) plane, as well as a sufficient amount of higher spatial frequency Γ(U,V) data, which corresponds to the non-central areas of the (U,V) plane. Since a diversity of interferometer baselines are required to provide the needed coverage in the (U,V) plane, the method of conventional aperture synthesis generally uses more than a single pair of receivers. For example, a conventional systems may use multiple receivers moving in controlled formations and covering pre-determined patterns to collect all the required Γ(U,V)data. When a single pair of receivers is used to provide a diversity of interferometer baselines, such a system generally is designed with the means to deliver a sufficient variation in the lengths and orientations associated with the data collecting interferometer baselines. The cost of a conventional aperture synthesis system depends on the amount of Γ(U,V) data that the system is designed to collect.
Prior art relating to correlation radiometry includes at least the Synthetic Thinned Aperture Radiometry (STAR), the Electronically Scanned Thinned Array Radiometer (ESTAR), and the Radiometric SAR (RADSAR). Prior art also includes systems that generally assume relatively short interferometer baselines, e.g., five wavelengths. Prior art usually relies on the Van Cittert-Zernike theorem and are limited by quasi-monochromatic assumption.
Hence it is highly desirable to improve imaging techniques.